The invention relates to a method for producing a rolling structure steel component.
Rolling structure is meant to encompass both rolling bearings and other parts of rolling structures, such as CVT and IVT disks and power rollers.
Such a steel is known and is for example described in the introduction of U.S. Pat. No. 4,992,111. This steel is known to have an improved bearing fatigue strength life. The high silicon content is restricted because of its costs and the probability of graphitisation problems during spheroidisation. Chromium is present for through hardening with a minimum of 1.30% by wt.
EP0721996A1 discloses in a very general way the composition for a high carbon bearing steel. Ranges given are generally broad, making it impossible to choose a specific composition.
For many applications it is a desire to decrease the size of components and structures. For example this is shown in the wheel bearing designs of a motor vehicle. The first step is to integrate to brake disk support and angular contact hub unit ball bearing. This is realised by selective surface induction hardening to obtain on the one hand sufficient hardness of the related raceways and on the other hand, sufficient toughness of the hub unit to take the drive and brake loads of the vehicle. However, further decreasing of such component will decrease the surface available for exchanging heat with the surroundings. This will result in an increased temperature of the related component.
A further example of increased temperature is due to down sizing of deep groove ball bearings in compact alternators.
Another example wherein higher temperatures usual are found is in IVT discs and power rollers. The maximum contact stress in these components can be as high as 4 GPa, which is far too high for known rolling bearing steels and the discs and power rollers would be succeptible to flaking at these stress levels. An example of such an IVT-transmission is given in the figures. In summary, the IVT must be designed for:
high contact fatigue strength,
high static load capacity,
high structural fatigue strength and fatigue crack arrest properties;
dimensional stability i.e. resistant to growth and axial clearance increase,
elevated temperature capability,
high tolerance to vibratory loads, and
resistance to false brinelling.
The invention aims to provide a steel as well as a method for producing components thereof, having a high surface strength and tough core with a high yield strength, stable microstructure with an elevated temperature capability.
According to the invention this is realized with a method for producing a rolling structure steel component, comprising 0.90-1.00% by wt. C; 0.95-1.05% by wt. Si, 0.25-0.45% by wt. Mn; 0.015% by wt S max., 0.025% by wt. P max.; 1.30-1.50% by wt. Cr and 0.17-0.25% by wt. Mo, balance Fe and impurities, wherein said steel component is through hardened by austenisation at 860xc2x120xc2x0 C., and tempered at 250xc2x120xc2x0 C.
Surprisingly, it has been found that the combination of silicon and molybdenum increases the high temperature rolling contact fatigue strength. A steel having this composition surprisingly does not show graphitisation problems in view of the high carbon percentage. Probably this is due to the narrow range of the silicon content. Because of the well defined Mo content, segregation is kept as reduced as possible.
Because it has been found that if the molybdenum content is too high in combination with high carbon excessive segregation is produced during liquid steel solidification in casting, an upper limit of 0.20% by wt. molybdenum. The higher molybdenum content is possible but will require longer homogenisation treatments and/or result in lost of hot workability during the steel making and hot forming operations for the manufacturing of rolling bearing and IVT components.
According to a further preferred embodiment of the invention 0.25% by wt. Ni or less is provided. Also, preferably 0.30% by wt. Cu or less is present. According to a further preferred embodiment 30 ppm Ti or less is present in the steel and the same applies to oxygen which should be present with 10 ppm at max. According to the invention selective surface hardening and more particular induction hardening can be used for selected surface areas.